The present invention relates to a method and apparatus for switching from a telecommunication line to a backup line in a telecommunication network system when a fault occurs on the telecommunication line, and more particularly to a method and apparatus of the kind described above in which a monitor time zone for monitoring the telecommunication line can be set by the line switching apparatus or a network control center in accordance with the service time of the network system, and the time of switching from the telecommunication line to the backup line is determined in accordance with the monitor time zone so set.
With the progress of telecommunication network technology, a telecommunication network system interconnecting computers located at distant places by a telecommunication line for the purpose of information processing is now widely used in this field. In such a network system, the reliability of the network is a matter of great importance, and various measures, including duplexing of the telecommunication line and associated equipment, are usually taken. For example, in a line switching method disclosed in JP-A-2-60337 by the present assignee, duplex telecommunication lines, that is, a usual telecommunication line and a backup line are provided, and, in the event of occurrence of a fault on the telecommunication line now in use, the telecommunication line is switched to the backup line by a line switching apparatus so as to ensure the reliability of the network. Further, in the prior art method, a network control center is connected to a plurality of line switching apparatuses located at different places for the purpose of centralized control of the line switching apparatuses so as to attain unitary line switching control from the network control center located at the remote place. In this case, a public switching network, such as an ISDN, is most frequently used as the backup line.
Suppose now that the prior art line switching method is used in a telecommunication network system in which a plurality of nodes are connected through line switching apparatuses to a telecommunication line and a backup line, and the service time of the network system is primarily determined. In the event of occurrence of a fault on the telecommunication line or shutdown of one of the nodes (due to node plan halt, fault occurrence, etc.) at a time other than the service time of the network system, the network control center instructs the associated one of the line switching apparatuses to switch from the faulty telecommunication line to the backup line, because the prior art line switching apparatus is always monitoring the telecommunication line. Consequently, the backup line that is the public switching network is used for telecommunication in a time zone other than the service time of the network system, thereby accruing an unnecessary extra cost due to the use of the telecommunication line in a time zone other than the service time of the network system.